This is a Phase 2/3 study that will take place in 2 stages. The primary objective of Stage 1 (Phase 2 dose-finding part)is to determine and select one dosage from three NewGam maintenance dosage arms in comparison with a placebo arm, based on the percentage of responders (response defined as a decrease, meaning improvement, in the adjusted INCAT disability score by at least 1 point). The selected NewGam dosage and placebo will be employed and compared in Stage 2.